netdev - Re: [PATCH v3 2/2] igb: Proactively round up to kmalloc bucket size

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Date:   Tue, 1 Nov 2022 14:37:17 -0700
From:   Kees Cook <keescook@...omium.org>
To:     "Ruhl, Michael J" <michael.j.ruhl@...el.com>
Cc:     "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Nguyen, Anthony L" <anthony.l.nguyen@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>,
        "intel-wired-lan@...ts.osuosl.org" <intel-wired-lan@...ts.osuosl.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] igb: Proactively round up to kmalloc bucket size

On Mon, Oct 31, 2022 at 08:42:36PM +0000, Ruhl, Michael J wrote:
> Looking at the size usage (from elixir), I see:
> 
> --
> 	if (!q_vector) {
> 		q_vector = kzalloc(size, GFP_KERNEL);
> 	} else if (size > ksize(q_vector)) {
> 		kfree_rcu(q_vector, rcu);
> 		q_vector = kzalloc(size, GFP_KERNEL);
> 	} else {
> 		memset(q_vector, 0, size);
> 	}
> --
> 
> If the size is rounded up, will the (size > ksize()) check ever be true?
> 
> I.e. have you eliminated this check (and maybe getting rid of the need for first patch?)?

Hi!

It looked like igb_alloc_q_vector() was designed to be called multiple
times on the same q_vector (i.e. to grow its allocation size over time).
So for that case, yes, the "size > ksize(q_vector)" check is needed. If
it's only ever called once (which is hard for me to tell), then no. (And
if "no", why was the alloc/free case even there in the first place?)

-Kees

-- 
Kees Cook